Methods and systems for production of an aqueous hypochlorous acid solution

ABSTRACT

A method for making an aqueous hypochlorous acid (HClO) solution includes electrolyzing a solution of sodium chloride to produce a solution of sodium hypochlorite; and producing the aqueous hypochlorous acid solution by adjusting a pH of the solution of sodium hypochlorite to a value within a range of 3 to 8 by adding a selected weak acid to the solution of sodium hypochlorite to produce a buffer including the selected weak acid and a salt of the selected weak acid.

FIELD

The present invention is directed to the area of methods and systems forthe production of an aqueous hypochlorous acid (HClO) solution. Thepresent invention is also directed to methods and systems for theproduction of hypochlorous acid by consumers.

BACKGROUND

Hypochlorous acid has been found to have bactericidal and virucidalproperties. Compared to sodium hypochlorite, which is often used assterilizing agent, hypochlorous acid is said to be over 80 times moreeffective. Hypochlorous acid can keep hands smooth, is eco-friendly, andcan be used directly to wash vegetables, fruits, and tableware. It canalso effectively eliminate many pesticide residues on the surface ofvegetables. Hypochlorous acid is approved by the US Food and DrugAdministration (FDA) and recommended by Japan's Ministry of Health,Labour and Welfare as a food-grade germicidal solution.

BRIEF SUMMARY

One embodiment is a method for making an aqueous hypochlorous acid(HClO) solution that includes electrolyzing a solution of sodiumchloride to produce a solution of sodium hypochlorite; and producing theaqueous hypochlorous acid solution by adjusting a pH of the solution ofsodium hypochlorite to a value within a range of 3 to 8 by adding aselected weak acid to the solution of sodium hypochlorite to produce abuffer including the selected weak acid and a salt of the selected weakacid.

In at least some embodiments, the aqueous hypochlorous acid solution hasno more than 500 ppm hypochlorous acid. In at least some embodiments,the method further includes adding a basic salt or a base to thesolution of sodium chloride. In at least some embodiments, the basicsalt or base reduces or absorbs chlorine gas generated during theelectrolysis. In at least some embodiments, the basic salt or base isselected from sodium bicarbonate, sodium carbonate, or sodium hydroxide.

In at least some embodiments, the selected weak acid is acetic acid andthe buffer is a combination of acetic acid and sodium acetate. In atleast some embodiments, the buffer in the aqueous hypochlorous acidsolution as a molar ratio of acetic acid to sodium acetate in a rangefrom 1:100 to 100:1.

In at least some embodiments, the method further includes diluting thesolution of sodium hypochlorite. In at least some embodiments, thesolution of sodium hypochlorite after the electrolyzing has at least 500ppm sodium hypochlorite. In at least some embodiments, the solution ofsodium hypochlorite after the electrolyzing has at least 1000 ppm sodiumhypochlorite.

In at least some embodiments, producing the aqueous hypochlorous acidsolution includes producing the aqueous hypochlorous acid solution byadjusting a pH of the solution of sodium hypochlorite to a value withina range of 4 to 6. In at least some embodiments, electrolyzing thesolution of sodium chloride includes electrolyzing the solution ofsodium chloride in an electrolysis cell including at least one positiveelectrode and at least one negative electrode without a membrane orseparator between the at least one positive electrode and the at leastone negative electrode. In at least some embodiments, the method furtherincludes transferring the solution of sodium hypochlorite from theelectrolysis cell after the electrolyzing and, after the transferring,receiving the selected weak acid in the electrolysis cell. In at leastsome embodiments, the selected weak acid removes calcium or magnesiumdeposits one the electrode surface. In at least some embodiments,un-softened water is used in the system for solution preparation and fordilution.

In at least some embodiments, the method further includes generating,storing or re-generating a solution of sodium hypochlorite in theelectrolysis cell while the hypochlorous acid product in the producttank is being consumed. In at least some embodiments, the method furtherincludes regenerating the sodium hypochlorite solution byre-electrolyzing the sodium hypochlorite solution after storing for apredetermined time period, wherein the predetermined time period is atleast twelve hours.

Another embodiment is a system for making an aqueous hypochlorous acid(HClO) solution. The system includes an electrolysis cell; a water tankor a coupling arrangement configured for coupling to an external watersource; an acid tank configured for receiving a selected weak acid; aNaCl tank configured for receiving an aqueous sodium chloride solution;a product tank; conduits individually coupling the water tank, acidtank, NaCl tank, and product tank to the electrolysis cell; and acontroller configured and arranged to perform actions when the NaCl tankcontains the sodium chloride solution, the acid tank contains theselected weak acid, water is in the water tank or the system is coupledto a water source using the coupling arrangement, the actions including:directing a portion of the aqueous sodium chloride solution from theNaCl tank to the electrolysis cell; electrolyzing the portion of thesolution of sodium chloride to produce a solution of sodium hypochloritein the electrolysis cell; directing the solution of sodium hypochloriteinto the product tank; and directing a portion of the selected weak acidin the acid tank into the solution of sodium hypochlorite to produce theaqueous hypochlorous acid solution by adjusting a pH to a value within arange of 3 to 8 by adding the selected weak acid to the solution ofsodium hypochlorite to form a buffer using the selected weak acid and asalt of the selected weak acid.

In at least some embodiments, the system is configured to produce theaqueous hypochlorous acid solution by providing acetic acid in the acidtank and a NaCl solution in the NaCl tank. In at least some embodiments,the system is further configured to produce the aqueous hypochlorousacid when a base or a basic salt is provided in the sodium chloridesolution to reduce production of chlorine gas, wherein the base or basicsalt is selected from sodium hydroxide, sodium carbonate, or sodiumbicarbonate.

In at least some embodiments, the electrolysis cell includes at leastone positive electrode and at least one negative electrode without amembrane or separator between the at least one positive electrode andthe at least one negative electrode. In at least some embodiments, atleast one of the at least one positive electrode or the at least onenegative electrode includes ruthenium and iridium. In at least someembodiments, at least one of the at least one positive electrode or theat least one negative electrode includes titanium.

In at least some embodiments, the system further includes a housing,wherein the electrolysis cell, water tank or coupling arrangement, acidtank, NaCl tank, product tank, and controller are disposed in thehousing. In at least some embodiments, the system further includes atleast one level indicator in at least one of the acid tank, NaCl tank,or product tank and coupled to the controller.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of one embodiment of system for making anaqueous HClO solution, according to the invention; and

FIG. 2 is a flowchart of a one embodiment of method for making anaqueous HClO solution, according to the invention.

DETAILED DESCRIPTION

The present invention is directed to the area of methods and systems forthe production of hypochlorous acid (HClO). The present invention isalso directed to methods and systems for the production of hypochlorousacid by consumers.

Many commercial methods of HClO production involve brine electrolysisusing a cell with a membrane. These commercial methods may be relativelycomplicated and only practical for commercial-scale applications.Production of hypochlorous acid has also been achieved on a commercialscale by adding an acid, such as hydrochloric acid, to sodiumhypochlorite (NaClO) via a precious pH control process. For small-scale,on-site, and on-demand applications, NaClO and HCl are not alwaysavailable. In addition, the precious pH control during the mixingprocess is required to prevent or reduce the generation of toxicchlorine gas and may be difficult in non-industrial arrangements. Asolution of sodium hypochlorite (NaClO) has been produced using a small,low cost electrochemical cell and a brine solution. However, sodiumhypochlorite is more toxic to the human body.

Methods and systems for generating an aqueous hypochlorous acid solutionare described herein. In at least some embodiments, the methods andsystems can be used by consumers to produce the hypochlorous acidsolution on a small-scale, on-site, or on-demand basis. In at least someembodiments, these methods and systems utilize readily availablecomponents, such as water, sodium chloride (NaCl), acetic acid (e.g.,vinegar), and either sodium bicarbonate (e.g., baking soda), sodiumcarbonate (e.g., soda ash or washing soda), or sodium hydroxide (e.g.,caustic soda or lye) to generate the hypochlorous acid. In at least someembodiments, the water is unsoftened.

The methods and systems described herein utilize the equilibrium of HClOand NaClO in aqueous solution where HClO is the dominate species insolutions with a pH between approximately 3 and approximately 7. In atleast some embodiments, the methods and systems are configured toproduce an aqueous hypochlorous acid solution with a pH in a range from3 to 8, a range from 3.5 to 7, a range from 4 to 6.5, or a range from 4to 6. HClO and NaClO typically reach equilibrium very quickly insolution. HClO exists stably at a pH range of at least 4 to 6.5.

To maintain the pH in the desired range, the aqueous hypochlorous acidsolution has a buffer that includes a weak acid and a salt of the weakacid and which maintains the pH in the desired range. In at least someembodiments, the buffer includes acetic acid and a salt of acetic acidsuch as sodium acetate, aluminum acetate, ammonium acetate, or potassiumacetate.

In at least some embodiments, the buffer can be formed by addition of aweak acid, such as acetic acid or the like, to the NaClO solution. Theaddition of acetic acid to the NaClO solution results in the formationof sodium acetate (which, in water, exists primarily in the form ofsodium and acetate ions). Thus, in at least some embodiments, the buffercan be prepared using household chemicals, such as acetic acid (e.g.,vinegar).CH₃COOH→CH₃COO⁻+H⁺pKa=4.76.

In water, CH₃COONa:CH₃COOH in a 1:1 molar ratio gives a solution with pHof approximately 4.76. CH₃COONa:CH₃COOH in a 10:1 molar ratio gives asolution with pH of approximately 5.76.

In at least some embodiments, the resulting aqueous hypochlorous acidsolution is stable for at least 5, 10, 15, 30, 60, or more days.

In at least some embodiments, a buffer of acetic acid and an acetatesalt, such as sodium acetate, can reliably keep the pH within a range of4 to 6.5 with a molar ratio of acetic acid to acetate salt in a rangefrom 1:100 to 100:1. In at least some embodiments, no accuratevolumetric control is needed for pH control due to the buffer.

In at least some embodiments, the addition of a soluble basic salt orbase may further facilitate formation of the buffer. For example, theaddition of soluble sodium bicarbonate, sodium carbonate, or sodiumhydroxide to the acetic acid may further the formation of sodium acetate(which, in water, exists primarily in the form of sodium and acetateions). In at least some embodiments, the soluble basic salt or base canbe household chemicals, such as sodium bicarbonate (NaHCO₃), commonlyknown as baking soda, sodium carbonate (Na₂CO₃), commonly known as sodaash or washing soda, or sodium hydroxide (NaOH), commonly known ascaustic soda or lye. For example,CH₃COOH+NaHCO₃→CH₃COONa+H₂O+CO₂

In at least some embodiments, adding the basic salt or base, such as,for example, NaHCO₃, Na₂CO₃, or NaOH, for the buffer into the NaClsolution can reduce Cl₂ generation during the electrolysis of NaCl.

In at least some embodiments, the components of the buffer areintroduced separately during the production of the HClO solution.

Methods and systems for preparing an aqueous HClO solution includegenerating a concentrated NaClO solution in an electrochemical devicefrom a solution containing NaCl, adjusting the pH of the NaClO solutionusing a buffer to produce the HClO solution, and diluting theas-produced NaClO or HClO to a desired concentration using water(preferably, unsoftened water).

The methods and systems include the electrochemical production of anaqueous NaClO solution from an aqueous NaCl solution according to thefollowing equation:NaCl+H₂O→NaClO+H₂

In at least some embodiments, the electrochemical production of aqueousNaClO is thought to proceed according to the following equations(although the invention does not rely on any particular mechanism orsequence of reactions):2NaCl+2H₂O→Cl₂+2NaOH+H₂ (electrolysis)Cl₂+H₂O→HClO+HCl (disproportionation)HClO+HCl+2NaOH→NaClO+NaCl+2H₂O (neutralization).

FIG. 1 illustrates one embodiment of a system 100 for production of anaqueous HClO (hypochlorous acid) solution. The system 100 includes awater source, such as water tank 102 or a coupling arrangement that canbe coupled to a stream of water from an external source; a sodiumchloride source, such as NaCl tank 104 containing an aqueous solution ofsodium chloride and, optionally, the basic salt or base for reducing thegeneration of Cl₂ gas; a weak acid source, such as acid tank 106containing the weak acid that forms the buffer; an electrolysis cell108; and a product tank 110. Any of the tanks can be replaced by anyother suitable reservoir or a coupling arrangement that can be coupledto a continuous or intermittent stream source.

The system 100 also includes a number of pumps 112 and various conduits116, such as tubing or the like to carry the reactants and othercomponents of the HClO solution, as well as the solution itself. Anyother suitable mechanisms, methods, or techniques for flowing thecomponents from the various sources to the electrolysis cell 108 and theproduct tank 110 can be used. For example, optional check valves may beused to prevent flow in the wrong directions. The HClO solution can beobtained at an outlet 118.

The system 100 also includes a control unit 122 that operates the systemin general including the pumps 112, as needed. In at least someembodiments, the control unit 122 can include one or more user operablecomponents, such as switches, buttons, a touchscreen, or the like topermit user control of the system 100.

The system 100 may include an optional filter 120 to filter the waterfrom the water tank 102. In at least some embodiments with a filter 120,there may also be a conduit from the water tank 102 to the valve 114that bypasses the filter 120. The pumps 112 can be individually anysuitable type of pump including, but not limited to, peristaltic pumps,diaphragm pumps, centrifugal pumps, or the like. The pumps 112 can beall the same type of pump or different types of pumps.

In at least some embodiments, one or more of the water tank 102, NaCltank 104, acid tank 106, or product tank 110 can include a level gauge124 to monitor the level of the respective solution or component in thattank. In at least some embodiments, the control unit 122 monitors thelevel gauge(s) 124 and, preferably, alerts a user if any level gaugedrops below a predetermined level or rises above a predetermined level.Level gauges 124 in the water tank 102, NaCl tank 104, or acid tank 106may indicate when additional source materials (e.g., water, NaCl, orweak acid) are needed. A level gauge 124 in the product tank 110 canindicate how much hypochlorous acid solution is presently available. Inat least some embodiments, the product tank 110 may include one levelgauge to monitor or warn for low fluid level and another level gauge tomonitor or warn for high fluid level.

In at least some embodiments, the product tank 110 (or any other tank)may include a pH gauge 126. In at least some embodiments, the controlunit 122 monitors the pH gauge 126. In at least some embodiments, thecontrol unit 122 may alert a user if any pH gauge is outside a desiredpH range and the control unit 122 may direct the user to dispose of thecontents of the product tank 110. In at least some embodiments, thecontrol unit 122 may automatically (or under user direction) pump theweak acid from the acid tank 106 or the solution containing the basicsalt from the NaCl tank 104 to adjust the pH.

Any suitable electrolysis cell 108 with two or more electrodes 109 a,109 b can be used. In at least some embodiments, the electrolysis cell108 does not include a membrane or separator between the electrodes 109a, 109 b. In at least some embodiments, the surface of the positiveelectrode(s) 109 a or negative electrode(s) 109 b contains ruthenium,iridium, or any combination thereof. In at least some embodiments, thesurface of the negative electrode(s) 109 b contains platinum. In atleast some embodiments, a bulk material of the positive and negativeelectrode(s) 109 a, 109 b is titanium, although any other suitablemetal, alloy, or combination thereof can be used.

In at least some embodiments, the system 100 can be disposed in a singlehousing 130. In at least some embodiments, the system 100 and housing130 can be portable. In at least some embodiments, as indicated above,instead of a water tank 102 (or other tank), the system 100 may includea coupling arrangement to couple to a streaming source of water (orother component). In at least some embodiments, one or more of the watertank 102, acid tank 106, NaCl tank 104, or product tank 110 can bedisposed outside the housing 130.

FIG. 2 is a flowchart of operation of the system to produce an aqueoussolution of HClO. In step 202, a portion of the NaCl solution from theNaCl tank 104 is pumped into the electrolysis cell 108. In at least someembodiments, the NaCl solution includes a basic salt or base (forexample, sodium bicarbonate, sodium carbonate, or sodium hydroxide) toreduce the generation of Cl₂ gas during the electrolysis. In at leastsome embodiments, the NaCl solution includes at least 5, 10, 20, 30, 50,100, 200, or 300 grams (or more) of NaCl per liter of water. In at leastsome embodiments, the NaCl solution includes, for example, 0.1, 0.25,0.5, or 1 gram of the basic salt or base per liter of the NaCl solution.In at least some embodiments, pre-prepared concentrated NaCl solutionand a weak acid solution are used to prepare the NaCl solution and theweak acid solution in tanks 104 and 106.

In step 204, the NaCl solution is electrolyzed in the electrolysis cell108 to produce an aqueous NaClO solution as described above. In at leastsome embodiments, the concentration of NaClO in the aqueous NaClOsolution produced in the electrolysis cell 108, prior to dilution, is atleast 500, 1000, or 5000 ppm. In at least some embodiments, the on-sitegeneration of high concentration NaClO via electrolysis of highconcentration NaCl is safe and efficient due to high concentration ofthe NaCl reactant and high solution conductivity. In at least someembodiments, the consumption of NaCl is no more than 0.4, 0.5, or 1 gramof NaCl per liter of aqueous HClO solution.

In step 206, the NaClO solution is pumped into the product tank 110 and,in step 208, the NaClO solution is diluted using water from the watertank 102. Steps 206 and 208 can be performed in any order so that theNaClO solution is diluted before or after being pumped into the producttank 110. In some embodiments, step 208 is skipped, and dilution occursduring later steps.

In step 210, after the electrolysis cell 108 is emptied, the weak acidfrom the acid tank 106 flows into the electrolysis cell 108 and,optionally, at least partially cleans the electrolysis cell of depositssuch as calcium carbonate or magnesium carbonate residue. In at leastsome embodiments, the weak acid in the acid tank 106 is acetic acid andhas a molarity of at least 0.1 or 0.3 M and may be in the range of 0.1to 16 M. In optional step 212, the weak acid is diluted in theelectrolysis cell 108 using water from the water tank 102.

In step 214, the acid, after optional dilution, is pumped into theproduct tank 110 and combined with the NaClO solution. The weak acidforms a buffer to adjust the pH to a range from 3 to 8, a range from 3.5to 7, a range from 4 to 6.5, or a range from 4 to 6 to produce theaqueous HClO solution. In at least some embodiments, the dilution of theNaClO or HClO solution during steps 208 and 214 is at least a factor of25, 50, 75, 100, 150, or 200 or more.

The user can remove the aqueous HClO solution through the outlet 118. Inat least some embodiments, the concentration of HClO in the aqueous HClOsolution, after dilution in the product tank 110, is in a range from 1to 500 ppm.

In at least some embodiments, the operation of the system 100 can becontinuous with the steps in FIG. 2 repeated on a continuous cycle. Inat least some embodiments, the system 100 can be programmed so that theoperation of the system 100 is repeated on a regular or periodic basis.In at least some embodiments, the system 100 can be programmed so thatthe operation of the system 100 is repeated based on measurements fromthe level gauge 124 in the product tank 110.

In at least some embodiments, the system 100 can store the concentratedNaClO solution or the diluted NaClO solution for periods of time(minutes, hours, or days) prior to introduction of the weak acid. In atleast some embodiments, the concentrated NaClO solution or the dilutedNaClO solution can be stored in the electrolysis cell 108, the producttank 110, or another storage tank (not shown) or any combinationthereof.

In at least some embodiments, the aqueous hypochlorous acid solution iskept in the product tank. When the aqueous hypochlorous acid solution isconsumed, a NaClO solution for the next batch of the aqueoushypochlorous solution is generated in the electrolysis cell 108 asdescribed above and is stored in the electrolysis cell until the NaClOsolution is needed to produce the next batch of the aqueous hypochlorousacid solution. Then, the NaClO solution is transferred to the producttank, diluted, and weak acid is added to produce more of the aqueoushypochlorous acid solution. In at least some embodiments, there may belimit (for example, 12 or 24 hours) to the length of time that the NaClOsolution can remain in the electrolysis cell before reverting, at leastin part, to an NaCl solution via the reaction below. In at lease someembodiments, the NaClO/NaCl solution in the electrolysis cell isre-charged every 24 hours as described above, or may be re-charged every12 to 120 hours, or may be re-charged before producing next batchhypochlorous product.2NaClO→2NaCl+O₂

The above specification provides a description of the manufacture anduse of the invention. Since many embodiments of the invention can bemade without departing from the spirit and scope of the invention, theinvention also resides in the claims hereinafter appended.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A system for making an aqueous hypochlorousacid (HClO) solution, the system comprising: an electrolysis cell; awater tank or a coupling arrangement configured for coupling to anexternal water source; an acid tank configured for receiving a selectedweak acid; a NaCl tank configured for receiving an aqueous sodiumchloride solution; a product tank; conduits individually coupling thewater tank, acid tank, NaCl tank, and product tank to the electrolysiscell; and a controller configured and arranged to perform actions whenthe NaCl tank contains the aqueous sodium chloride solution, the acidtank contains the selected weak acid, water is in the water tank or thesystem is coupled to a water source using the coupling arrangement, theactions including: directing a portion of the aqueous sodium chloridesolution from the NaCl tank to the electrolysis cell; electrolyzing theportion of the aqueous sodium chloride solution to produce a solution ofsodium hypochlorite in the electrolysis cell; directing the solution ofsodium hypochlorite into the product tank; and directing a portion ofthe selected weak acid in the acid tank into the solution of sodiumhypochlorite to produce the aqueous hypochlorous acid solution byadjusting a pH to a value within a range of 3 to 8 by adding theselected weak acid to the solution of sodium hypochlorite to form abuffer using the selected weak acid and a salt of the selected weakacid.
 2. The system of claim 1, wherein the system is configured toproduce the aqueous hypochlorous acid solution by providing acetic acidin the acid tank and a NaCl solution in the NaCl tank.
 3. The system ofclaim 2, wherein the system is further configured to produce the aqueoushypochlorous acid solution when a basic salt or a base is provided inthe aqueous sodium chloride solution to reduce production of chlorinegas.
 4. The system of claim 1, wherein the electrolysis cell comprisesat least one positive electrode and at least one negative electrodewithout a membrane or separator between the at least one positiveelectrode and the at least one negative electrode.
 5. The system ofclaim 4, wherein at least one of the at least one positive electrode orthe at least one negative electrode comprises ruthenium and iridium. 6.The system of claim 4, wherein at least one of the at least one positiveelectrode or the at least one negative electrode comprises titanium. 7.The system of claim 1, further comprising a housing, wherein theelectrolysis cell, water tank or coupling arrangement, acid tank, NaCltank, product tank, and controller are disposed in the housing.
 8. Thesystem of claim 1, further comprising at least one level indicator in atleast one of the acid tank, NaCl tank, or product tank and coupled tothe controller.
 9. The system of claim 1, wherein the system isconfigured to produce the aqueous hypochlorous acid solution which hasno more than 500 ppm hypochlorous acid.
 10. The system of claim 3,wherein the basic salt or base is selected from sodium bicarbonate,sodium carbonate, or sodium hydroxide.
 11. The system of claim 1,wherein the selected weak acid is acetic acid and the buffer is acombination of acetic acid and sodium acetate.
 12. The method of claim11, wherein the buffer in the aqueous hypochlorous acid solution has amolar ratio of acetic acid to sodium acetate in a range from 1:100 to100:1.
 13. The system of claim 1, wherein the actions further comprisediluting the solution of sodium hypochlorite.
 14. The system of claim 1,wherein the controller is configured so that the solution of sodiumhypochlorite after the electrolyzing has at least 500 ppm sodiumhypochlorite.
 15. The system of claim 1, wherein the controller isconfigured so that the solution of sodium hypochlorite after theelectrolyzing has at least 1000 ppm sodium hypochlorite.
 16. The systemof claim 1, wherein producing the aqueous hypochlorous acid solutioncomprises producing the aqueous hypochlorous acid solution by adjustinga pH of the solution of sodium hypochlorite to a value within a range of4 to
 6. 17. The system of claim 1, wherein the actions further comprisetransferring the solution of sodium hypochlorite from the electrolysiscell after the electrolyzing and, after the transferring, receiving theselected weak acid in the electrolysis cell.
 18. The system of claim 1,wherein the actions further comprise generating or regenerating asolution of sodium hypochlorite in the electrolysis cell while theaqueous hypochlorous acid solution in the product tank is beingconsumed.
 19. The system of claim 1, wherein the actions furthercomprise regenerating a solution of sodium hypochlorite in theelectrolysis cell while the aqueous hypochlorous acid solution in theproduct tank is being consumed.
 20. The method of claim 19, whereinregenerating the solution of sodium hypochlorite comprisesre-electrolyzing the solution of sodium hypochlorite after storing for apredetermined time period, wherein the predetermined time period is atleast twelve hours.
 21. The system of claim 1, wherein the actionsfurther comprise storing a solution of sodium hypochlorite in theelectrolysis cell while the aqueous hypochlorous acid solution in theproduct tank is being consumed.
 22. The system of claim 1, wherein theelectrolysis cell comprises at least one positive electrode and at leastone negative electrode.